Extragalactic Gamma-ray Background from Star-forming Galaxies: Will Empirical Scalings Suffice?

TL;DR

This paper critically examines the use of empirical scalings to estimate the contribution of star-forming galaxies to the extragalactic gamma-ray background, highlighting potential biases and the need for more sophisticated models.

Contribution

It identifies key biases in empirical scaling methods and emphasizes the importance of improved models for accurately estimating gamma-ray contributions from star-forming galaxies.

Findings

Biases in empirical scalings can significantly affect gamma-ray background estimates.

Large parameter spaces can overproduce the gamma-ray background, constraining models.

More complex, physics-based models are needed for accurate predictions.

Abstract

Despite the influx of unprecedented-quality data from the Fermi Gamma-Ray Space Telescope that have been collected over nine years of operation, the contribution of normal star-forming galaxies to the extragalactic gamma-ray background (EGRB) remains poorly constrained. Different estimates are discrepant both their underlying physical assumptions and their results. With several detections and many upper limits for the gamma-ray fluxes of nearby starforming galaxies now available, estimates that rely on empirical scalings between gamma-ray and longer-wavelength luminosities have become possible and increasingly popular. In this paper we examine factors that can bias such estimates, including: a) possible sources of nontrivial redshift dependence; b) dependence on the choice of star-formation tracer; c) uncertainties in the slope and normalisation of empirical scalings. We find that such biases can be significant, pointing towards the need for more sophisticated models for the star-forming galaxy contribution to the gamma-ray background, implementing more, and more confident, physics in their buildup. Finally, we show that there are large regions of acceptable parameter space in observational inputs that significantly overproduce the gamma-ray background, implying that the observed level of the background can yield significant constraints on models of the average cosmic gamma-ray emissivity associated with star formation.